Field of the Invention
The present invention relates to a pressure sensor which detects pressure fluctuation based on a pressure difference.
Description of the Related Art
In the related art, as a pressure sensor (differential pressure sensor) which detects pressure fluctuation, for example, a pressure sensor including a substrate which has a through-hole or a recess portion, a storage container which has a vent hole, and a piezoelectric element which is provided inside the storage container and is cantilevered by the substrate to be vibratable inside the through-hole or the recess portion is known (see Japanese Unexamined Patent Application, First Publication No. H04-208827 (hereinafter, referred to as Patent Document 1)).
According to this pressure sensor, the piezoelectric element vibrates in response to pressure fluctuation transmitted to the inside of the storage container through the vent hole, and pressure fluctuation can be detected based on change in voltage of the piezoelectric element.
On the other hand, this type of pressure sensor is designed to detect pressure fluctuation within a frequency band according to the purpose of the pressure sensor. At this time, for example, in the pressure sensor of Patent Document 1, pressure fluctuation detection sensitivity is determined by the shape of the piezoelectric element, the volume of the through-hole or the recess portion, the flow rate between the through-hole or the recess portion and outside gas, and the like, and in particular, largely depends on the shape of the piezoelectric element.
However, since the piezoelectric element includes electrode films and the like on both surfaces of a piezoelectric body, reduction in thickness is rarely achieved, and a large amount of deformation is rarely secured. Accordingly, a piezoelectric sensor disclosed in Patent Document 1 has a limit on pressure fluctuation detection and is not enough to detect minute pressure fluctuation.
It is considered that the upper limit frequency of pressure fluctuation detectable by the piezoelectric sensor is near the resonance frequency of the piezoelectric element. In regard to a lower limit frequency, currently, no design guide has been obtained. For this reason, when the pressure fluctuates slowly, it is difficult to accurately understand what level of frequency is detectable, or the like.
Therefore, in order to obtain the lower limit frequency of pressure fluctuation detectable by the piezoelectric sensor, there is only a way to produce a plurality of types of piezoelectric sensors based on various design parameters and to combine a plurality of detections results of these piezoelectric sensors, thereby measuring the lower limit frequency. Accordingly, it is practically difficult to set the lower limit frequency detectable by the piezoelectric sensor to an arbitrary value.